The present invention relates to an arrangement for recirculation of exhaust gases of a supercharged combustion engine and for cooling the gases during recirculation.
The technique known as EGR (Exhaust Gas Recirculation) is a known means of leading part of the exhaust gases from a combustion process in a combustion engine back, via a return line, to an inlet line for supply of air to the combustion engine. A mixture of air and exhaust gases is thus supplied via the inlet line to the engine's cylinders in which the combustion takes place. Adding exhaust gases to the air causes a lower combustion temperature which results inter alia in a reduced content of nitrogen oxides NOx in the exhaust gases. This technique is used for both Otto engines and diesel engines.
The amount of air which can be supplied to a supercharged combustion engine depends on the pressure of the air but also on the temperature of the air. In order to supply as large an amount of air as possible to the combustion engine, the compressed air is cooled in a charge air cooler before it is led to the combustion engine. The compressed air is cooled in the charge air cooler by means of ambient air. The compressed air can thus be cooled to a temperature which exceeds the temperature of the surroundings by only a few degrees. In cases where EGR technology is used, the returned exhaust gases are cooled by means of a so-called EGR cooler. Conventional EGR coolers use as cooling medium the coolant in the vehicle's ordinary cooling system for cooling the combustion engine. Conventional EGR coolers are therefore subject to the limitation that the exhaust gases cannot be cooled to a lower temperature than the temperature of the coolant in the cooling system, which during normal operation is of the order of 70-90° C. The cooled exhaust gases are therefore usually at a definitely higher temperature than the cooled compressed air when they mix in the inlet line to the combustion engine. The mixture of exhaust gases and air which is led to the combustion engine will therefore be at a higher temperature than the compressed air which is led into a supercharged combustion engine not provided with recirculation of exhaust gases. The performance of a supercharged combustion engine equipped with EGR will thus be somewhat inferior to that of a corresponding supercharged combustion engine not equipped with EGR.